Messy findings: planets encounter a violent world.

Rocky planets such as Earth are born through countless acts of violence--the collision and merging of many smaller bodies. A new study reveals that some planets continue to take a beating hundreds of millions of years after they've formed.

The evidence comes from an infrared survey of 266 youthful stars, revealing that 71 of them have disks of dusty debris. The disks are a sign that newborn planets are being clobbered by asteroids and that the asteroids--planet-formation leftovers--are banging together (SN: 10/,9/04, p. 227) and making dust that glows at infrared wavelengths.

Using NASA's Spitzer Space Telescope, which detects disks dimmer than other infrared telescopes can, researchers now have dozens of debris disks to study. The images show that some stars are swaddled by bright disks even if they're several hundred million years old. Many theorists had expected that by that time, most of the dust would have dissipated--either blown outward or dragged inward by the star's radiation.

"The only way to produce as much dust as we are seeing in these older stars is through huge [recent] collisions," says George Rieke of the University of Arizona in Tucson. He presented the findings in an Oct. 18 NASA briefing, and his team reports details in an upcoming Astrophysical Journal.

The collisions indicated by the Spitzer data may be similar to a significant smashup in our own solar system. Theorists have proposed that Earth's moon arose when a Mars-size projectile struck our planet.

The new study suggests that "the collisions of the kind that might have produced the moon are the rule rather than the exception," comments Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Rieke's team examined the disks of A stars, which are two to three times as massive as the sun. The stars studied lie between 20 and 500 light-years from Earth and are from a few million to about 800 million years old. Observing at a wavelength of 24 micrometers, the telescope focused on a region within the disks where terrestrial planets might form. That area is 10 to 20 times as for from each star as Earth is from the sun.

The study reveals that even for A stars of the same age, the amount of dust varies considerably. The variability suggests that "disks don't all decay away in some gradual way," says study coauthor Karl R. Stapelfeldt of NASA's Jet Propulsion Laboratory in Pasadena, Calif. Instead, he asserts, the disks evolve "spasmodically" perhaps all but vanishing and then reappearing after a major collision generates a fresh supply of dust.

Future infrared telescopes, as well as some large near-infrared telescopes now in use, may indicate whether the stars' planets are small and rocky like Earth or large and gaseous like Jupiter, he says.